In the production of miniaturized objects such as miniature devices including integrated circuits and microelectronics for semiconductor and display applications, the tools and auxiliary structures used in their manufacture, as well as the miniature objects themselves have to be examined carefully. Optical methods of examining these tools and objects are non-destructive and frequently preferred over other approaches. Hence, advances in optical examination of miniature features including patterns composed of adjacent features are important.
In many cases miniature devices are made by photolithographic techniques. In a typical application of the photolithographic technique, a layer of photoresist is deposited on a substrate or other device layer and then exposed to radiation of appropriate wavelength through a patterning mask. Of course, the masks themselves also need to be appropriately patterned with miniature features to be able to perform their function and are thus themselves a class of miniature devices that has to be examined.
Now in photolithography certain regions of the photoresist layer are exposed and others are not, according to the pattern defined in the patterning mask. Exposing the photoresist to radiation changes its solubility. After exposure, solvent is used to remove regions of higher solubility photoresist, leaving regions of “hardened” photoresist at sites on the device layer as dictated by the patterning mask. The “hardened” photoresist remains to protect the underlying material from removal during a subsequent etching step or other suitable material removal procedure. After etching the photoresist is discarded. In this manner, a feature is created in the device based on the pattern defined in the mask.
Clearly, the photoresist layer must be accurately patterned to form features to the exacting specifications for miniature devices. It is therefore desirable to monitor the photolithographic process at various stages and on a periodic basis. For example, it would be desirable to measure the thickness of the photoresist layer and examine the pattern to determine feature sizes. The thickness can be measured by subjecting the photoresist to light with a wavelength in the range of 190 to 1000 nm and measuring the reflected light. The reflected radiation may be correlated to photoresist thickness. The general principle of this measurement technique is that the measured light reflected from a substrate is modulated by constructive and destructive optical interference from an overlaying semitransparent material such as the photoresist. For more information see Chopra, K. L., Thin Film Phenomena, p. 99 (McGraw Hill, 1969). The periodicity of the reflectance spectra can also be used to determine optical properties, such as the refractive index n of the substrate.
Measurement of the pattern or features is a more difficult procedure. For example, in a typical application, the pattern consists of a plurality of stripes and spaces, e.g., a line and space pattern. These types of patterns are frequently encountered in forming diffractive elements such as lenses or gratings in semiconductors or glass, forming fluid flow microchannels in silicon, and in general for providing a variety of mechanical features in a substrate. In measuring stripe widths and separations the prior art techniques have typically relied on scanning electron microscopy (SEM). Unfortunately, SEM is a destructive and very time-consuming examination method.
Methods such as atomic force microscopy (AFM) and profilometry are also viable for examining features or patterns of features. However, both of these methods are very time consuming and they require special test structures in most cases.
The patterning masks used to create resist lines often themselves contain features. Of particular interest are Alternating Phase Shift Masks (AAPSMs), which are often quartz or fused silica plates etched with trenches in repeating patterns. This creates an interference condition between light passing through the etched and un-etched regions of the mask, leading to complete amplitude cancellation in regions that would normally have been exposed. In this way an AAPSM can be used to pattern features in the resist that are smaller than the wavelength of light used to expose the resist. Accurate metrology control of the dimensions of these features is critical, since in a typical application using 248 nm wavelength light, approximately 13 Å difference in trench depth is enough to change the phase shift by 1 degree.
In addition to AFM, SEM, and profilometry the prior art offers interferometric techniques for measuring high-precision patterns, such as those encountered in AAPSMs. Unfortunately, because of the inherent limitations of AFM, SEM and profilometry already mentioned, these techniques are not satisfactory for examining AAPSMs. Interferometric techniques are too expensive, and require special test structures. Furthermore, the test features have to be large enough so that reference and measurement beams can be fully covered by two different uniform areas respectively. These test features often do not reflect the phase shift characteristics of the features to be printed on the mask. In addition, in most cases the test features have to be transparent. This condition prevents the measurement from being performed at the early stages of mask processing when an opaque metallic layer, such as Cr, is frequently present.
For more information on AAPSMs and methods for examining them the reader is referred to Cynthia B. Brooks, et al., “Process Monitoring of Etched Fused Silica Phase Shift Reticles”, Proceedings of the SPIE, 22nd Annual BACUS Symposium on Photomask Technology and Management, Sep. 30-Oct. 4, 2002, Monterey, Calif., USA; Alessandro Callegari and Katherina Babich, “Optical Characterization of Attenuated Phase Shifters”, SPIE, Vol. 3050, pp. 507-514; as well as Pieter Burggraaf, “Lithography's Leading Edge, Part I: Phase Shift Technology”, Feb. 1992, pp. 43-47.
More recently, attempts have been made to measure patterns using scatterometry. In this technique, a pattern is subjected to light, such as from a laser, typically having a single wavelength. This light is usually directed toward the pattern at some angle to the normal. The light reflected from the pattern at various diffracted orders is measured. It may be possible to use such data to obtain quantitative information about the pattern. However, scatterometry is very sensitive to small changes in the profile of the pattern, and requires relatively sophisticated correlation work to relate the reflected radiation to the features of a pattern. The computational effort required to correlate the reflected radiation to the pattern is very high since the convergence criteria for these solutions take a very long time to compute. In addition, the measured pattern must be periodic. Other examples of characterization methods pertaining to photolithography and equipment suitable for practicing such methods are described in U.S. Pat. Nos. 5,867,276; 5,363,171; 5,184,021; 4,866,782 and 4,757,207. There are still other types of scatterometry, which measure the specularly reflected light as a function of wavelength, as taught in U.S. Pat. Nos. 6,483,580; 5,963,329; 5,739,909 and 5,607,800.
Of these references U.S. Pat. No. 5,607,800 to Ziger teaches a method and arrangement for characterizing features of a patterned material on an underlayer. His approach is based on selecting an appropriate wavelength range where the patterned material absorbs more radiation than the underlayer. In other words, substrate or underlayer is more reflective than the pattern or surface features in this wavelength range. The reflectance spectrum uniquely identifies the pattern and can be used to study similar patterns by comparing their reflectance spectra. Unfortunately, just as in the case of scatterometry, when patterns vary this comparison-based approach can not be used effectively to study patterns which differ substantially from each other.
U.S. Pat. No. 6,100,985 to Scheiner et al. teaches a method for measuring at least one desired parameter of a patterned structure having a plurality of features. In this method a measurement area, which is substantially larger than a surface area of the structure defined by the grid cycle, is illuminated by an incident radiation of a preset substantially wide wavelength range. The light component that is substantially specularly reflected from the measurement area is detected, and measured data representative of photometric intensities of each wavelength within the wavelength range is obtained. The measured and theoretical data are analyzed and the optical model is optimized until the theoretical data satisfies a predetermined condition. Upon detecting that the predetermined condition is satisfied at least one parameter of the structure is can be calculated.
A still more recent teaching for optically determining a physical parameter of a pattern made up of features is taught in U.S. Pat. No. 6,327,035 to Li et al. This teaching goes further than Scheiner et al. by examining various response light fractions including an underlayer light fraction and a feature light fraction and using reference physical parameters of the underlayer. The response light can be either transmitted or reflected and the reference physical parameters of the underlayer are either known a priori or determined.
U.S. Pat. No. 6,340,602 to Johnson et al. teaches a method for measuring a parameter associated with a portion of a sample having one or more structures with at least two zones each having an associated zone reflectance property. The at least two zones are illuminated with broadband light, the reflected light is measured and a measured reflectance property is fit to a model. The model mixes the zone reflectance properties to account for partially coherent light interactions between the two zones.
Although Johnson's approach attempts to address coherence issues between the zones, it does not take into account the interactions between the broadband light and the substrate. More precisely, in this approach the substrate is assumed to be opaque and only lateral incoherence between the zones themselves is treated. In most cases, however, substrates on which features or zones are measured are at least partially transparent over a portion or even the entire broadband spectrum of the incident broadband light. Thus, by leaving out the complex interactions between the illuminating light, the zones and the substrate, Johnson is not able to provide a method that can be used for measuring zones or features on semi-transparent and transparent substrates.
In fact, the problem of optically examining features and patterns on underlayers or substrates that are at least semi-transparent or fully transparent has eluded a satisfactory solution because of its complexity. This complexity is partly due to the large series of internal reflections and transmissions affecting the response light obtained from the substrate and features. What is more, the response light is not only conditioned by the multiple internal reflections and transmissions within the substrate and features to be examined, but also by coherent and incoherent interactions between reflected and/or transmitted response light from the substrate and the various features.